1. Field of the Invention
The present invention relates to electric circuits for operating fluorescent lamps, and, more particularly to ballast circuits for compact fluorescent lamps.
2. Description of the Prior Art
Fluorescent lamps are a conventional type of lighting device which are gas charged devices that provide illumination as a result of atomic excitation of low-pressure gas, such as mercury, within a lamp envelope. The excitation of the mercury vapor atoms is provided by means of a pair of arc electrodes mounted within the lamp. In order to properly excite the mercury vapor atoms, the lamp is ignited and operated at a relatively high voltage, and at a relatively constant current. The excited atoms emit invisible ultraviolet radiation. The invisible ultraviolet radiation in turn excites a fluorescent material, e.g., phosphor, that is deposited on an inside surface of the fluorescent lamp envelope, thus converting the invisible ultraviolet radiation to visible light. The fluorescent coating material is selected to emit visible radiation over a wide spectrum of colors and intensities.
As is known to those of skill in the art, a ballast circuit is commonly disposed in electrical communication with the lamp to provide the elevated voltage levels and constant current required for fluorescent illumination. Typical ballast circuits electrically connect the fluorescent lamp to line alternating current and convert this alternating current provided by the power transmission lines to the constant current and voltage levels required by the lamp.
Fluorescent lamps have substantial advantages over conventional incandescent lamps. In particular, the fluorescent lamps are substantially more efficient and typically use 80 to 90% less electrical power than an equivalent light for output incandescent lamps.
For these reasons, these lamps have been widely used in a number of applications, especially in commercial buildings where the unusual shape and size (in contrast to incandescent bulbs) is either not a disadvantage or is actually an advantage.
In view of the significant advantages of the fluorescent tubes, it would seem to be a natural to largely replace use of the incandescent lamp in the home environment, especially now that compact fluorescent tubes are available.
However, to date, these lamps have several serious disadvantages which have limited their use. These disadvantages include:
1. The ballast circuit unlike an incandescent bulb, presents a non-linear load to the A.C. line. Typically the power factor which measures the phase relationship of the current and voltage of a conventional ballast circuit is about 0.4 which is an undesirable level. One prior solution to the ballast circuit problem is to employ an electronic ballast circuit which electrically is more efficient. However, these ballast circuits require a large number of electrical components which increases the cost of the fluorescent lamp. Further, the addition of these electrical components cause harmonic distortion problems and provide a lower than desired power factor.
2. Fluorescent lamps have been relatively large, both because of the lamp itself but also because of the space required to house the ballast circuit. As a result, contemporary fluorescent lamps cannot readily replace many incandescent lamps used in the home and elsewhere.
3. Dimmable fluorescent lamps suffer from a number of compromises. Common problems are flickering and striations, e.g., alternating bands of illumination and non-illumination across the fluorescent lamps, in the dimmed conditions, uneven non-gradual dimming, a small range of dimming, and high cost of the dimming circuit.
4. Conventional ballasts emit unacceptable levels of electromagnetic interference (EMI) and radio-frequency interference (RFI). The high levels of interference often make the fluorescent lamp unacceptable near radios, televisions, personal computers and the like.
5. Although the fluorescent tube itself has a very long life, the ballast, particularly ballasts capable of dimming the fluorescent tube, have suffered from excessive failures in the field. In addition, many dimmable fluorescent lamp ballasts suffer catastrophic failure if the ballast is plugged into line voltage without a fluorescent tube in the circuit.
6. Some prior art ballast circuits require a large ferrite core inductor to be placed between the lamp and the input power circuit to provide a selected degree of electrical isolation between the power transmission lines at the input and the lamp, while allowing the conduction of the necessary current levels to the fluorescent lamp. Despite the fact that these ballast circuits provide the desired current and voltage levels they do so at the price of the electrical efficiency of the ballast circuit.